The present invention generally relates to devices and methods for measuring properties of fluids. More particularly, this invention relates to a microfluidic device equipped with a microchannel through which a fluid flows and means for ascertaining properties of the fluid while flowing through the microchannel.
Fluid delivery devices, systems, and methods involve technologies under constant development. Examples of fluid delivery systems of particular current interest include drug infusion systems and fuel cell systems, both of which require devices capable of delivering relatively small amounts of a fluid. While fuel cells have been used for many years to provide electrical power, currently there is increased interest for their use in consumer products such as automobiles, computers, cellular phones, personal digital assistants (PDA's), camcorders, and other portable devices. Fuel cell systems typically employ a small electrically powered fluid pump to deliver fluids to various parts of the system, such as water, fuels, and fuel cell solutions, examples of which include mixtures of water and fuels such as methanol, ethanol, ethylene glycol, isopropyl alcohol (IPA), formic acid, sulfuric acid, gasoline, diesel fuel, and other organic liquids. The solution is delivered to a fuel cell, such as a reformed fuel cell, direct methanol fuel cell (DMFC), or proton exchange membrane (PEM) fuel cell (or PEMFC), which can be adapted to provide power to a vehicle or other device that requires electrical power.
As well known in the art, in a fuel cell system it is important to know the concentration of a fuel in fuel cell solution to optimize the efficiency of the system. For example, DMFC's often employ a fuel cell solution of methanol mixed with water to reduce membrane crossover problems and boost the efficiency of the fuel cell. If the methanol concentration is too high, crossover problems can occur, whereas low methanol concentrations reduce the power output of the fuel cell. Consequently, various concentration sensors for fuel cell systems have been proposed, including electrolytic, refractometer, ultrasonic, electrochemical, electromagnetic, and electromechanical sensors. An example is an electromechanical system disclosed in commonly-assigned U.S. patent application Publication No. 2006/0213552 to Sparks et al., which makes use of a Coriolis-based fluid sensing device preferably of a type disclosed in commonly-assigned U.S. Pat. No. 6,477,901 to Tadigadapa et al., whose contents relating to the fabrication and operation of a Coriolis-based sensor are incorporated herein by reference. Sparks et al. teach that chemical concentrations, including those of fuel cell solutions, can be measured by sensing changes in fluid density as a fluid sample flows through a microchannel within a resonating tube of a Coriolis-based fluid sensing device.
A fluid sensing device 10 of a type disclosed by Tadigadapa et al. and Sparks et al. is represented in FIGS. 1 and 2. The device 10 is represented as including a micromachined tube 14 extending from a base 28 on a substrate 12 and having a freestanding portion 16 above a surface 18 of the substrate 12. Drive and sensing electrodes 22 and 24 are located on the surface 18 beneath the freestanding portion 16 of the tube 14, and bond pads 32 (only one of which is shown) are provided for transmitting input and output signals to and from the device 10. The drive electrode 22 can be, for example, capacitively coupled to the tube 14 for capacitively (electrostatically) driving the freestanding portion 16 at or near resonance, while the sensing electrodes 24 sense (e.g., capacitively, optically, etc.) the deflection of the tube 14 relative to the substrate 12 and provide feedback to enable the vibration frequency induced by the drive electrode 22 to be controlled with appropriate circuitry. With a fluid entering the device 10 through an inlet port 26 and flowing through an internal passage 20 within the tube 14, the freestanding portion 16 can be vibrated at or near resonance by the drive electrode 22 to ascertain certain properties of the fluid, such as flow rate and density, using Coriolis force principles. In particular, as the freestanding portion 16 is driven at or near resonance by the drive electrode 22, the sensing electrodes 24 sense a twisting motion of the freestanding portion 16, referred to as the Coriolis effect. Because the twisting motion is more readily detectible along the parallel legs of the freestanding portion 16, the sensing electrodes 24 may be positioned along the entire lengths of the legs. The degree to which the freestanding portion 16 deflects during a vibration cycle as a result of the Coriolis effect can be correlated to the mass flow rate of the fluid flowing through the tube 14, while the density of the fluid is proportional to the frequency of vibration at resonance. Notable advantages of the device 10 include the extremely miniaturized scale to which it can be fabricated and its ability to precisely analyze very small quantities of fluids. In FIG. 2, the device 10 is schematically shown as enclosed by a cap 30 to allow for vacuum packaging that further improves the performance of the device 10 by reducing air damping effects.
During fuel cell power generation processes, carbon dioxide and other gases are generated that can form bubbles within the fuel cell solution. Any air dissolved in the solution can also form bubbles under high temperature or low pressure conditions. Bubbles present in a liquid can cause errors in chemical concentration outputs based on density, as well as density measurements made by measuring speed of sound (ultrasonic measurements), refractive index, and other methods. Sensing errors can also occur over time as a result of films and residues building up on sensing elements such as tubes and windows, resulting in an offset shift in the chemical concentration output. For resonating tubes of the type employed by Sparks et al., bubbles present in the liquid being evaluated will increase the resonant frequency of the resonating tube, and build up of a film or residue on the internal surfaces of the tube will lower the resonant frequency of the tube, resulting in errors in density measurements.
The ability to detect potential measurement errors of the types noted above is complicated by other potential sources of sensor output drift, including imperfections due to manufacturing variations and defects, particles (or other second phases) in the fluid being sensed, differences in materials that lead to different responses to temperature and mechanical stress, charge buildup, and others. Therefore, while sensors of the type taught by Tadigadapa et al. and used by Sparks et al. have proven to be extremely precise in their ability to measure properties of fluids, further improvements capable of addressing the above-noted issues would be desirable.